In various types of signal and communication systems for use in railroad and mass and/or rapid transit operations, it is customary practice to employ cab signals to control the speed of a vehicle or train as it moves along its route of travel. Normally, the cab signals which are received on board the vehicle or train are in the form of coded carrier waveforms. That is, the carrier signal is selectively coded or modulated by one of a plurality of code or pulse rates. Each code or pulse rate signifies or represents a given maximum speed or velocity at which a vehicle or train is permitted to travel along any given block or section of the track. In actual operation, the coded carrier signals are normally conveyed to the track rails by a transmitter connected thereto and are picked up by inductive receiver coils which are mounted forward of the front axle of the lead vehicle or locomotive. The picked up signals are amplified, demodulated, shaped, and filtered, and then the recovered signals are applied to the speed command decoding unit.
In typical speed control systems for railroad as well as mass and/or rapid transit operations, a speed command decoding unit has been carried on the vehicle involved. For example, in a cab signaling speed control system, the cab signals are received from the rails and are applied to the cab signaling receiver for processing. By comparing the decoded speed command signal with the actual vehicle speed signal produced by an axle driven generator, it is possible to determine whether a vehicle is proceeding at the appropriate authorized speed for any given section of track. In such operations, it is mandatory that any overspeed condition be immediately detected and that the necessary measures, such as braking, be instituted to correct the situation. A further requirement of such operation is that under no circumstance should a critical circuit or component failure simulate a true condition. Thus, every vital circuit including filter circuits of the vehicle-carried speed command decoding unit must operate in a failsafe fashion; that is, the electronic filter should not be capable of passing signals having frequencies substantially different from those of a band of signals adjacent a preselected frequency.
In other control systems applicable to railroad operations, the objective has been simply to provide a coded control signal indicative of the presence of a train or other vehicle in a given track section along the right of way, where different code rates are used in different track sections. In these control systems, a decoding scheme has also been deployed.
In the past, passive series resonant circuit decoders were often used as the frequency selective element in vehicle-carried decoding units and train detection units. FIG. 1, attached hereto, depicts a commonly used passive series resonant circuit decoder 10 wherein a code is input into decoder driver 12 and relay driver 14, which send an output signal to a relay. Due to the fact that the frequencies are quite low, e.g., 1.25 to 21.5 Hz, the capacitors (C) and particularly the inductors (L) are very large, heavy and expensive. In addition to the resonant circuit, a decoder driver 12 with sufficient drive capability is needed to drive the circuit. This drive circuit may be a medium power transistor driver circuit or any other suitable arrangement, such as code following relay driving a master step-up transformer which is used as the input to the series resonant circuit.
Accordingly, it is a primary object of the present invention to replace the previously noted components with an all electronic decode system which is much smaller and less costly.
There have been various attempts to overcome the disadvantages of the large, heavy, and expensive passive series resonant decoders. Most prominent amongst the alternative devices are the active bandpass filters. Examples of active bandpass filters used in decoding railway signals can be found in the following U.S. Pat. Nos.: 4,001,710 (Darrow), issued Jan. 4, 1977; 4,290,027 (Parker), issued Sept. 15. 1981; 3,725,802 (Darrow), issued Apr. 3, 1973; and 4,368,440 (Darrow), issued Jan. 11, 1983.
U.S. Pat. No. 4,001,710 discloses a low frequency selective amplifier circuit having a feedback loop including an R-C twin-T network which is imperfectly nulled to a particular frequency and having an emitter follower for isolating the series resistance branch of the R-C twin-T network from the load of the selective amplifier circuit so that an output signal is produced when and only when an input signal having the particular frequency is present and no critical component or circuit failure exists.
U.S. Pat. No. 4,290,027 discloses an active bandpass filter circuit comprising a first section including a twin-T filter network, a power source connected to the twin-T network, a second section including a first resistor connected between a common or reference point and the input of the twin-T network, and a second, feedback resistor connected from the output to the input of the twin-T network such that the loop gain is less than one.
U.S. Pat. No. 3,725,802 discloses an electronic filter circuit including a feedback amplifier. The feedback path of the amplifier includes a twin-T network which is imperfectly nulled to only provide regeneration at a preselected frequency so that an output signal is only available during the presence of a signal having the preselected frequency and in the absence of a critical component or circuit failure.
U.S. Pat. No. 4,368,440 discloses a low-pass filter employing a selected one of a plurality of transistor gates and switches for establishing a circuit path from a source of AC signals to the low-pass filter and for determining an upper frequency signal passing limit for the low-pass filter. This circuit also includes a four terminal timing capacitor.
Most conventional active bandpass filters typically use operational amplifiers with feedback for the active filter which are very difficult to make failsafe due to changes in gain or oscillations caused by the various failure modes which may occur. The active element for these filters is normally a unity gain amplifier, which under the wrong failure mode could increase its gain. This could cause oscillations or change the filter response so that it might pass a code rate frequency that is outside of its normal bandpass.
The present invention provides a failsafe decoder system which includes passive components having failure modes that will cause the filter to fail in the safe direction. Furthermore, the novel active bandpass filter includes high-pass and low-pass filters connected in series which individually can only fail in such a way as to reduce the band of frequencies which may pass therethrough during component failure. The active bandpass filter is connected to a level detector which is also failsafe. The level detector produces a proper output only if both an upper and a lower threshold voltage is crossed, which is dependent upon appropriate D.C. bias existing in the filter.
It should be noted that the combination of an electronic high-pass and low-pass filter has been known. For example, such a combination has been employed in musical applications. (See U.S. Pat. No. 3,475,623 (Moog), issued Oct. 28, 1969). However, it will be appreciated that this patent, as well as others, have no application to railway speed control decoders or the like.
The present invention also provides many additional advantages which shall become apparent as described below